Ultrasonication-assisted spray ionization mass spectrometry for on-line monitoring of organic reactions

Article abstract of DOI:10.1039/c0cc02629h

A straightforward on-line monitoring of organic reactions by ultrasonication-assisted spray ionization mass spectrometry (UASI MS) is demonstrated in this work.

Full text of DOI:10.1039/c0cc02629h

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Ultrasonication-assisted spray ionization mass spectrometry for on-line
monitoring of organic reactionsw
Tsung-Yi Chen, Chin-Sheng Chao, Kwok-Kong Tony Mong* and Yu-Chie Chen*
Received 17th July 2010, Accepted 28th September 2010
DOI: 10.1039/c0cc02629h
A straightforward on-line monitoring of organic reactions by
ultrasonication-assisted spray ionization mass spectrometry
(UASI MS) is demonstrated in this work.
Ultrasonication has been employed to accelerate organic
syntheses1–8 and enzymatic reactions.9–11 For example, high
biodiesel yields can be obtained through the transesterification
of vegetable oils in the presence of alcohol and base catalyst
under ultrasonication within a much shorter time than those
obtained through conventional approaches.6,7 Under ultrasonic
mixing, smaller droplet sizes of alcohol/oil dispersions accelerate
the reaction, leading to high transesterification yield.8 The
hydrolytic activity of lipase10 and tryptic digestion of proteins11
can be enhanced dramatically under ultrasound irradiation. The
main advantages of ultrasonication-assisted reactions include
Fig. 1 Cartoon representation of the configuration of the on-line
short reaction time, high yields, and ease of use.
monitoring of ultrasonication-assisted reactions by UASI MS.
Recently, we explored a new atmospheric pressure ionization
mass spectrometry, so-called ultrasonication-assisted spray
ionization mass spectrometry (UASI MS),12 which has been
and simultaneously monitored by the MS. Thus, monitoring
reactions through this approach is very straightforward. In this
study, we employed the UASI MS to monitor ultrasonication-
assisted reactions in real time.
demonstrated to be suitable for the analysis of small and large
biomolecules. The background in the UASI mass spectra is
rather low, allowing for a good signal to noise (S/N) ratio12
compared with that obtained using the conventional electro-
spray ionization mass spectrometry (ESI MS). The good S/N
ratio is due to the fact that a high electric field is not required
for the generation of sample spray in the UASI MS approach.
Therefore, the background ions derived from electrochemical
reaction are eliminated. In the UASI MS, a capillary is
placed into an Eppendorf tube containing the sample solution
subjected to an ultrasonicator used to provide a sufficient
driving force to direct the liquid sample solution from the
inlet to the tapered outlet of the capillary, leading to the
generation of the ultrasonic spray. The analyte ions derived
from the ultrasonic spray are directly detected using MS. As
ultrasonication has been used in assisting organic reactions,
there are no compromises in coupling the UASI MS
with ultrasonication-assisted reactions (Fig. 1). Reactants
dissolved in a suitable solvent are placed in an Eppendorf
tube, which has been subjected to an ultrasonicator. Once the
ultrasonicator is switched on, ultrasonication-assisted organic
reaction is carried out. The reaction intermediates and
products continually run through the capillary, spraying out
from the capillary outlet with the assistance of ultrasonication
A transesterification reaction called the Zemplen reaction
is generally used in deblocking reactions in carbohydrate
chemistry.13 Ultrasound has been successfully employed in
promoting the reaction.14 We selected a Zemplen reaction as
the model reaction for on-line monitoring using the UASI MS.
Esters are versatile groups in hydroxyl functions during
oxidation, peptide coupling reactions, or glycosylations and
are typically deprotected by base-catalyzed solvolysis. Among
the large number of ester protecting groups, benzoyl ester is less
susceptible to the undesirable acyl migration; consequently, it is
widely used in organic synthesis. The removal of the benzoate
group is usually accomplished using Zemplen conditions, where
a base catalyst is employed to catalyze a transesterification
process. When multiple benzoyl functions are present in the
same molecule, an analytical method that can assess the
removal rate of each individual function is imperative. In doing
so, a regioselective deprotection may be realized. Thus, the
real-time monitoring UASI MS is applied to monitor the
Zemplen deprotection of a model substrate, namely, 4-O-benzyl-
2,3,6-tri-O-benzoyl thioglucopyranoside (A). Sodium methoxide
(MeONa) is used as the catalyst for the Zemplen reaction. The
synthesis of reactant A was described in ESI.w Scheme 1 illustrates
the Zemplen reaction. The expected intermediates and the final
product are labeled B, C, and D, respectively.
Department of Applied Chemistry, National Chiao Tung University,
Hsinchu 300, Taiwan. E-mail: yuchie@mail.nctu.edu.tw,
tonymong@cc.nctu.edu.tw; Fax: +886 3-5723764;
Tel: +886 3-5131527
w Electronic supplementary information (ESI) available: Experimental
details and supplementary results are provided. See DOI: 10.1039/
c0cc02629h
Prior to performing the on-line monitoring of the Zemplen
reaction, the reaction products obtained from off line were
collected at different reaction time points and subjected to ESI
MS for analysis. For comparison, the reaction was performed
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m/z 503 was revealed in the same mass spectrum. After the
reaction went on for 42 min, the ESI mass spectrum of the
reaction sample resembled that of Fig. 2d (Fig. 2e). The product
ion derived from the sodiated product through the loss of three
benzoyl functional groups was not observed in the mass
spectrum after reacting for 65 min (Fig. 2f). The intensity of
the peak at m/z 607 continued to elevate in the mass spectrum
(Fig. 2f), whereas the peak at m/z 711 dominated the mass
spectra after the reaction had stood for 65 min (Fig. 2f).
As mentioned earlier, ultrasonication has been demonstrated
to be effective in accelerating organic reactions; thus, we also
performed the Zemplen reaction under ultrasonication. The
reaction products were collected at specific time points.
Fig. 3a–f display the ESI mass spectra of the reaction products
obtained at the reaction time of 0, 12, 27, 35, 42, and 65 min,
respectively. At the beginning, the peak at m/z 711 corresponding
to sodiated reactant A dominated the mass spectrum (Fig. 3a).
The peaks at m/z 437, 620, and 647 derived from the solvent
and background were also observed in the same mass
spectrum. After the reaction had proceeded for 12 min, the
peak at m/z 711 still dominated the mass spectrum, whereas a
weak peak at m/z 607 derived from the loss of a benzoyl
function group was revealed in the mass spectrum (Fig. 3b). In
addition, a peak at m/z 503 resulting from the loss of two
benzoyl functional groups also appeared in the same mass
spectrum. The peaks at m/z 503 and 607 were slightly raised
after the reaction continually reacted for 27 and 35 min
(Fig. 3c and d). As the reaction time reached 42 min, the peak
at m/z 607 dominated the mass spectrum. An extra peak at m/z
399 with a low intensity was also revealed in the same mass
spectrum (Fig. 3e). The peak at m/z 607 remained as the base
peak after the reaction went on for 65 min (Fig. 3f). However,
the peak at m/z 711 corresponding to sodiated reactant A
reduced substantially; while the intensities of the peaks at m/z
399 and 503 increased to a slight extent. The results presented
in Fig. 3 indicate that the reaction was effectively accelerated
under ultrasonication compared with those obtained by the
conventional procedures as shown in Fig. 2.
Scheme
1 Zemplen deprotection of 4-O-benzyl-2,3,6-tri-O-benzoyl
thioglucopyranoside (A) in the solvent of methanol/acetonitrile (1/1, v/v).
The catalyst was MeONa. ACN and MeOH stand for acetonitrile and
methanol, respectively.
under different conditions, including vortex-mixing and
Fig. 2a–f display the ESI mass spectra of the reaction products
collected at the reaction time points of 0, 12, 27, 35, 42, and
65 min, respectively. The peaks at m/z 437, 620, and 647 were
presumably derived from the solvent background and observed
in each mass spectrum. The peak at m/z 711 derived from
sodiated reactant A dominated the mass spectrum in Fig. 2a
prior to reaction. A weak peak at m/z 607 representing the
sodiated intermediate resulting from the loss of a benzoyl
function group of reactant A began to appear in the mass
spectrum after reacting for 12 min (Fig. 2b). After the reaction
had proceeded for 27 min, the peak at m/z 607 was slightly
raised in the mass spectrum as seen in Fig. 2c. A very weak peak
at m/z 503, a sodiated peak, representing the loss of two benzoyl
functional groups from the reactant began to appear in the
same mass spectrum. As the reaction time reached its 35th min,
the peak at m/z 607 became enhanced (Fig. 2d). The peak at
Fig. 3 ESI mass spectra of the samples obtained from the Zemplen
reaction using 4-O-benzyl-2,3,6-tri-O-benzoyl thioglucopyranoside (A)
prepared in the solvent of methanol/acetonitrile (1/1, v/v) as the
product; MeONa was used as catalyst. The reaction was conducted
under ultrasonication. The samples were collected for ESI MS analysis
at the following reaction time points: (a) 0, (b) 12 min, (c) 27 min, (d)
35 min, (e) 42 min, and (f) 65 min.
Fig. 2 ESI mass spectra of the samples obtained from the Zemplen reaction
using 4-O-benzyl-2,3,6-tri-O-benzoyl thioglucopyranoside (A) prepared in
the solvent of methanol/acetonitrile (1/1, v/v) as the product; MeONa was
used as catalyst. The reaction was conducted under vortex-mixing. The
samples were collected for ESI MS analysis at the following reaction time
points: (a) 0, (b) 12 min, (c) 27 min, (d) 35 min, (e) 42 min, and (f) 65 min.
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monitoring time increased. Such results are attributed to con-
tinuous consumption of the reactant A in the reaction course.
The peak at m/z 607 (B), which is a sodiated ion, attributed to the
loss of the benzoyl function group at the C-6 primary position,
was also observed in the same mass spectrum. Fig. 4d displays
the mass spectrum obtained from the averaged mass spectra
between 20 and 25 min in Fig. 4a. Aside from the peaks at
m/z 711 and 607 observed in Fig. 4b, there was an extra peak that
appeared in the mass spectrum at m/z 503, which is a sodiated
peak corresponding to the loss of two benzoyl functional groups
of reactant A. As the reaction time increased, the peak at
m/z 399, which represents the loss of three benzoyl functional
groups, began to appear in the mass spectra. Fig. 4e displays the
EIC at m/z 607, 503, and 399 plotted in purple, green, and blue,
respectively. The ion current for m/z 607 increased continually,
whereas the ion current for m/z 503 remained the same. The ion
current for m/z 399, a sodiated ion, became noticeable after
reaction for B44 min. Fig. 4f presents the mass spectrum
obtained from the averaged mass spectra acquired between
50 and 55 min in Fig. 4a. As expected, the peak at m/z 399
was observed in the mass spectrum except for the peaks at
m/z 503, 607, and 711. The peak at m/z 607 dominated the mass
spectrum. This result indicates that the regioselective removal of
C-6 benzoate ester in substrate A is made possible by the real-
time monitoring of the reaction with the use of UASI MS
method. Additionally, the background ions at m/z 437, 620,
and 647 appearing in conventional ESI mass spectra in Fig. 2 and
3 are not observed in Fig. 4. The results demonstrate that
background in UASI mass spectra is rather low.
Fig. 4 Zemplen deprotection of 4-O-benzyl-2,3,6-tri-O-benzoyl thio-
gluco-pyranoside (A) prepared in the solvent of methanol/acetonitrile
(1/1, v/v). The catalyst was MeONa. (a) TIC chromatogram obtained
during the on-line monitoring of the Zemplen reaction using the UASI
MS; (b) UASI mass spectrum obtained from the averaged mass
spectra acquired between the reaction time at 3 and 10 min in Panel
a; (c) EIC at m/z 711; (d) UASI mass spectrum obtained from the
averaged mass spectra acquired between 20 and 25 min in Panel a; (e)
EIC at m/z 607, 503, and 399 plotted in purple, green, and blue,
respectively; and (f) UASI mass spectrum obtained from the averaged
mass spectra acquired between 50 and 55 min in Panel a.
After demonstrating that the Zemplen reaction accelerated
effectively under ultrasonication, we monitored the reaction with
the UASI MS in real time. The setup of the UASI MS and the
fabrication of the capillary tip are briefly stated in ESIw, which
are mainly based on the approach described in the previous
study.12 The combination of UASI MS with the ultrasonic-
assisted reaction was straightforward; a capillary filled with
solvent was placed into the reaction vial subjected to an ultra-
sonicator, and the tapered outlet of the capillary was placed close
to the inlet of a mass spectrometer. The products generated from
the ultrasonic-assisted reaction were monitored readily through
the mass spectrometer upon switching on the ultrasonicator.
We initially filled out a capillary with acetonitrile/methanol
(1/1, v/v) solvent and placed it into the reaction vial containing
reactant A prepared in acetonitrile/methanol (1/1, v/v). The
reaction vial was subjected to ultrasonication, whereas the
tapered outlet of the capillary was placed close to the inlet of
an ion trap mass spectrometer (Fig. 1). The catalyst, MeONa,
was added into the reaction vial when the ultrasonicator was
switched on. The ions generated from the capillary outlet via
ultrasonic spray were monitored using the mass spectrometer
in real time. Prior to powering the ultrasonicator on, the mass
spectrometer was employed to acquire ions during the first
2 min (Fig. 4a). The mass spectrometer detects low intensities
of ions derived from a volatile solvent at the beginning. Once
the ultrasonicator was switched on, the total ion current (TIC)
increased dramatically as seen in Fig. 4a. After monitoring the
ions for 56 min, the ultrasonicator was switched off, and
the intensity of the ions suddenly decreased. During the first
3–10 min, the sodiated reactant A at m/z 711 marked in
red dominated the average mass spectrum (Fig. 4b). Fig. 4c
presents the extracted ion chromatogram (EIC) at m/z 711
during the on-line monitoring of the Zemplen reaction by the
UASI MS. The intensity of the EIC at m/z 711 decreased as the
In conclusion, we have demonstrated a very straightforward
approach for on-line monitoring of organic reactions using the
UASI MS as detection method. There are no compromises in
combining ultrasonication-assisted organic reactions and the
UASI MS analysis. This approach can be further used for
various types of organic reactions assisted by ultrasonication
such as enzymatic reactions and polymer degradations.
We thank the National Science Council (NSC) of Taiwan
for the financial support of this work.
Notes and references
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Chem. Commun., 2010, 46, 8347–8349 8349